The present invention relates to a yarn feed roller assembly for a tufting machine, and also to a method of controlling the pile height of individual stitches in a tufting machine.
U.S. Pat. No. 5,182,997 discloses a yarn feed roller assembly with two longitudinally extending drive rollers, each of which is rotated at a different speed. Associated with each end of yarn is a pivotal arm having a pair of yarn feed wheels each associated with a respective drive roller. A control mechanism is arranged to move the pivotal arm to bring one or other of the feed wheels into contact with a corresponding drive roller so that the yarn is driven by one or other of the drive rollers. When the faster drive roller is used, the yarn feed speed is high thereby tufting a high pile. On the other hand, when the low speed roller is used, the yarn feed speed is reduced and a low pile is tufted. The machine allows the pile height of each individual stitch to be controlled to be either high or low. This individual control is known as a full repeat scroll.
As a development of this, to provide greater patterning flexibility, a machine referred to as a three pile height full repeat scroll has been developed by the applicant. In place of the two drive rollers, this machine uses three drive rollers each of which is driven at a different speed. In a similar way, by selecting with which of the three drive rollers an end of yarn is engaged during a stitch, three different pile heights can be formed.
In order to obtain even greater patterning flexibility, it has been proposed to replace the drive and yarn feed rollers with an individual servo motor for each end of yarn. Thus, instead of three different pile heights, this machine is capable of producing a tufted carpet, in which each stitch has a pile height which can be of any height between maximum and minimum limits. However, this greater flexibility in patterning capability is extremely costly given the number of servo motors required.
According to the present invention, a yarn feed roller assembly for a tufting machine comprises a first drive roller arranged to be rotatably driven, and a plurality of actuators, each being arranged to bring an end of yarn selectively into driving engagement with the first drive roller; characterized by control means containing pattern data relating to the required pile height of each stitch, the control means being arranged to calculate from this the required proportion of the stroke for which the yarn is required to be driven by the first drive roller to achieve the required pile height, and to control the movement of each actuator so that an end of yarn is driven by the first drive roller for the required proportion of the needle stroke.
This machine provides the same patterning capabilities of continuously variable pile heights that are obtainable with the machine which has a servo motor for each end of yarn. However, it has been estimated that a machine according to the present invention can be produced for significantly less than the cost of the machine with servo motors.
In the broadest sense, the yarn is driven only by the first drive roller and is engaged with this roller for as long as is necessary to generate the required pile height. In this case, the yarn has to be gripped when it is not being driven by the drive roller to prevent the yarn from being dragged into the backing cloth by the needles. However, a preferred option is to provide a second drive roller which is arranged to rotate at a slower speed than the first drive roller, wherein each actuator is arranged to switch an end of yarn such that it is driven either by the first or the second roller to obtain the required pile height. Thus, in order to produce higher pile heights, the actuator will leave the yarn in contact with the first drive roller for a longer proportion of the needle stroke, while to produce lower pile heights, the actuator will leave the yarn in contact with the second drive roller for a longer period. The twin roller arrangement allows the yarn to be fed constantly during the needle stroke, rather than the stop/start motion provided by the single drive roller arrangement. This allows full control of the yarn during the whole needle stroke.
Although the first and second rollers allow any pile height between upper and lower limits to be produced, the invention could be performed with a yarn feed roller assembly having three or more drive rollers all driven at different speeds. The presence of more than two rollers does not allow a greater variety of pile heights to be generated. However, it will have some benefit in that it can reduce the frequency with which the actuator switches between rollers. For example, a yarn feed roller assembly with three drive rollers will be able to produce three different pile heights without having to switch from one roller to another during a needle stroke, it may be that the majority of the carpet can be produced using these three pile heights. Nevertheless, when required, the actuators can switch the yarn from one roller to another during the needle stroke hence producing stitches with intermediate heights.
Each actuator may comprise a pivotable arm having a pair of yarn feed wheels one of which is arranged to selectively press the yarn into engagement with the first drive roller, and the other of which is arranged to selectively press the yarn into engagement with the second drive roller as the arm is pivotally moved. However, preferably, the actuator is provided by an arm having a yarn feed wheel about which the yarn is engaged, and an intermediate wheel which drivingly engages with the yarn feed wheel, the arm being movable such that the intermediate wheel can be selectively brought into driving engagement with either of the first and second drive rollers. Thus, as the yarn engages with the yarn feed wheel and not the intermediate wheel which selectively engages the two drive rollers, the possibility of the yarn being dragged as the intermediate wheel is moved from one drive roller to the other is minimized. As a consequence of this, the clearance between the intermediate wheel and the drive rollers can be reduced, thereby improving the response time of the machine and hence, the accuracy of the pile height.
In an alternative arrangement, the actuator is provided by an arm having a yarn feed wheel, the arm being moveable such that the yarn feed wheel can be selectively brought into driving engagement either with the first or second drive rollers, and means for guiding the yarn around a portion of the yarn feed wheel which does not contact the drive rollers, the yarn feed wheel having a surface which engages with the yarn so as to provide a frictional drive for the yarn. This also provides an arrangement in which the yarn is not fed between the yarn feed wheel and the driver roller.
The present invention also extends to a method of controlling the pile height of individual stitches in a tufting machine comprising, a drive roller arranged to be rotatably driven and a plurality of actuators each being arranged to bring an end of yarn selectively into contact with the drive roller the method comprising the steps of:
determining the required pile height of a particular stitch from pattern data;
determining the proportion of the needle stroke for which the yarn will need to be in contact with the drive roller to achieve the required pile height; and
operating the actuator to bring the yarn into contact with the drive roller for the required proportion of the needle stroke.